A process wherein a carbohydrate-containing feedstock is dehydrated is known from WO2007/104514. In this known process a fructose- or glucose-containing starting material is contacted with an alcohol in the presence of an acid catalyst. The catalyst may be homogeneous or heterogeneous, e.g., a solid. This known process is of particular interest for the preparation of ethers of 5-hydroxymethylfurfural. The process is suitable for being conducted in a continuous mode. Thereto, a starting material comprising the carbohydrate and alcohol was fed to a reactor containing the acid catalyst. In the reactor the temperature was 125 to 300° C. This process was exemplified by experiments wherein about 10 grams of glucose or fructose-containing feedstock was dissolved per liter alcohol. Although the conversion and selectivity of the reaction was satisfactory, a drawback of this process is constituted by the limited solubility of the carbohydrate in the alcohol. Therefore, only diluted solutions of the carbohydrate in alcohol could be reacted in a continuous flow reactor without clogging the heterogeneous catalyst.
In U.S. Pat. No. 2,917,537 a process for the manufacture of levulinic acid and its esters has been described. In this process a carbohydrate is reacted with a lower alkanol and a non-reacting diluent in the presence of a strong acid catalyst at a temperature of 150 to 250° C. The diluent is insoluble in water and has a heat of vaporisation less than that of water and suitably less than that of the alkanol. It may be selected from a group consisting of hydrocarbons, chlorinated hydrocarbons, ethers and esters with an atmospheric boiling point below 190° C. In an example 100 pounds of sugar is mixed with 160 pounds of methanol and 240 pounds of benzene in the presence of one pound of sulphuric acid. It will be evident to the skilled person that the reaction mixture already initially comprises a dispersion of solid sugar in the methanol/benzene mixture. The mixture obtained is heated to 215° C. to carry out the conversion to the methyl ester of levulinic acid. The start of the process with such a dispersion makes it difficult to carry out the reaction in a continuous manner and impossible if the continuous process is conducted in a fixed bed reactor.
It is an object of the invention to provide a process wherein a carbohydrate-containing feedstock and an alcohol can be processed in one homogeneous phase, whilst the concentration of the carbohydrate-containing feedstock can be brought to a level that would make the conversion economically attractive.
It has now surprisingly been found that at temperatures below 100° C. a homogeneous solution of a carbohydrate in an alcohol in attractive concentrations can be obtained if the carbohydrate and the alcohol are present in a mixture that also comprises an acid catalyst.